Sound is transmitted by pressure oscillations in air. A microphone is a pressure sensor designed to sense very small pressure oscillations across the audio frequency range (20 Hz-20 kHz). Typically, a compliant diaphragm is designed to deflect in proportion to sound pressure. The deflection is, in-turn, measured in a number of ways (capacitively, optically, or piezoelectrically) to ultimately produce an output voltage in proportion to the sound pressure. Piezoelectric materials produce a voltage when strained. When piezoelectric materials are patterned on a pressure sensitive diaphragm, the deflection of the diaphragm due to sound pressure strains the diaphragm and a voltage is produced by the piezoelectric material. This is one example of how sound is transduced into an electrical signal.
Since their entry into the market, electrostatic microelectro-mechanical systems (MEMS) microphones have become one of the highest growth areas for MEMS, growing from less than 300 million units shipped in 2007 to over 1 billion units shipped in. Apple's iPhone 4 product alone contains three electrostatic MEMS microphones including two in the body of the phone and a third in the mobile headset.
Directional microphones with a high degree of directivity sense sound with high sensitivity in preferred directions, while being relatively insensitive to sound in other directions. Directional microphones may be used in hearing-aids, for example, to avoid the common “cock-tail” party problem. Directivity enables a hearing-aid wearer to listen to a speaker of interest with high sensitivity, while rejecting ambient background noise that would otherwise degrade speech intelligibility, i.e., directional microphones improve signal to noise ratio. Clearly, directional microphones have potential to greatly improve speech intelligibility and signal clarity for a wide suite of consumer electronic devices given the pervasive use of audio in our daily lives (e.g., smartphones, laptop computers, Bluetooth earpieces, hearing aid devices, etc.).
Rotational microphones biologically inspired by a special type of parasitoid fly (Ormia ochracea) have been demonstrated by Miles et al. at SUNY Binghamton and Degertekin et al. at Georgia Tech (R. N. Miles, Q. Su, W. Cui, M. Shetye, F. L. Degertekin, B. Bicen, C. Garcia, S. Jones, and N. Hall, “A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea,” J Acoust Soc Am, vol. 125, pp. 2013-26, April 2009, and B. Bicen, S. Jolly, K. Jeelani, C. Garcia, N. Hall, F. L. Degertekin, Q. Su, W. Cui, and R. Miles, “Integrated optical displacement detection and electrostatic actuation for directional optical microphones with micromachined biomimetic diaphragms,” IEEE Sensors Journal, pp. 1933-1941, 2009). In addition to offering a very compact pressure gradient microphone with experimentally verified “figure-8” directivity, laboratory prototypes demonstrated a simultaneous 10 dB lower noise floor and factor of 10-times reduction in size compared to state of the art low-noise miniature microphones used in hearing aids. Demonstrations to date, however, have relied on complex optical readout approaches which face challenging packaging and manufacturing hurdles. Integration of multiple sensors on a single die to realize co-located pressure gradient measurements will also prove challenging due to alignment tolerances between optical and mechanical components. Designing for low power consumption is yet another challenge with optical readout methodologies.